What are some common mistakes to avoid when implementing custom error pages through Web.config?

Avoid displaying detailed error messages in production environments. Ensure custom error pages handle all possible error scenarios gracefully. Do not create recursive redirects (an error page that redirects to another error page). Test your error pages thoroughly under various error conditions. Also, don't include sensitive information in your error messages. Always log errors to aid in debugging and monitoring the application. And test your configuration across different environments (development, staging, production).